Production of sheet glass

ABSTRACT

Novel processes for the production of a smooth translucent glass sheet comprise grinding at least one face of a clear glass sheet using a conventional abrasive such as sand. Preferably the clear glass sheet is a short of clear float glass. The process may be carried out using a conventional glass grinding and polishing facility operated in a manner that the ground sheet is ground but not polished. The product preferably has a satin finish and a roughness Ra of less than 5.0 μM and is sufficiently translucent as to be useful in privacy glazing. The ground glass products may have a haze value of from 80 to 90% and an Ra value of from 1.0 μM to 3.0 μM are believed to bc novel.

CONTINUING APPLICATION DATA

This application is a divisional application of U.S. patent applicationSer. No. 09/528,292, filed on Mar. 17, 2000, now U.S. Pat. No.6,510,708. U.S. patent application Ser. No. 09/528,292 is herebyincorporated by reference as if set forth in its entirety herein.

FIELD OF THE INVENTION

This invention relates to translucent smooth glass sleets, to novelprocesses for their production and to novel methods of operating a glassgrinding and polishing production facility so as to produce translucentsmooth lass sheets.

DESCRIPTION OF THE PRIOR ART

Translucent glass sheets find use in a variety of applications in whichthere is a need for light transmission coupled with a desire for adegree of privacy. Products of this type can be made by a variety ofprocesses. Shot blasting processes are used to produce a translucentsheet product having a relatively rough surface. The roughness of thesurface means that these products are not acceptable for allapplications. Acid etching processes are used and produce a producthaving a relatively smooth surface. However acid etching processespresent obvious safety and environmental hazards and are therebyexpensive to operate. Rollin processes using an appropriate roller mayalso be used but produce a product with a relatively rough surface.

Clear smooth glass sheets are currently produced almost exclusively bythe float glass process. Prior to the advent of the float glass processin the late 1950's clear smooth glass sheets were produced by grindingand polishing relatively rough glass plates. Typical grinding andpolishing processes are described at page 705 of “The Handbook of GlassManufacture” Volume II, edited by Fay V Tooley and published by Booksfor Industry, 777 Third Avenue, New York, N.Y. 10017. The use of glassgrinding and polishing facilities to produce sheet glass is now largelyconfined to the production of glass products which cannot be producedusing the float process. The most prominent example is the production ofpolished wired safety glass

SUMMARY OF THE INVENTION

We have now discovered a novel method for the production of atranslucent glass sheet which comprises grinding at least one surface ofa clear glass sheet with a suitable abrasive. These, methods may be andpreferably axe carried out using an existing facility for the grindingand polishing of sheet glass. The clear glass sheet may be subjected toall or part of the grinding process but is not polished. The product ofthese grinding processes preferably have a smoothness close 10 that ofacid etched glass. The ground surface has characteristics whichdistinguish those products from the acid etched glass sheets. Certain ofthose ground glass sheets are believed to be novel and these sheetscomprise a second aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

From one aspect this invention provides a process for the production ofa translucent smooth glass sheet which comprises grinding at least onesurface of a clear flat lass sheet under controlled conditions so as toproduce a translucent sheet having a smoothness better than 50 μM Ra.

RA is a measure of roughness of the glass surface which is thearithmetic mean of the absolute departures of the roughness profile fromthe mean line. The translucent glass sheet will generally b, less smooththan the clear sheet from which it is produced. In is specification allmeasurements of the surface of the translucent glass sheet were obtainedusing a KSA Tencor P10 profiler (both to produce a three-dimensionalimage and a two-dimensional one line scan profile) unless specificallystated otherwise. A stylus having a radius of 0.1 μM was employed usinga loading of 0.5 mg. The short range filter was maintained at theinstrument's default setting of 8.3 μM corresponding to a scan rate of50 μM per second and the data point sampling rate of 50 Hz. Nolong-range filter was employed. Three-dimensional images were recordedover an evaluation area of 250 μM×250 μM. Fifty scans were collected perimage giving a scan line spacing of 5 μM. The two dimensional profileswere recorded over an evaluation length of 4 mm.

The ground glass sheets produced by the processes of this inventionpreferably have Ra values falling within the range 0.05 μM to 5.0 μM.Preferably they will have an Ra value falling within the range 0.05 μMto 3.0 μM or most preferably 0.1 μM to 2.0 μM.

The preferred type of clear flat glass sheet for use in the processes ofthis invention is a sheet of float glass. Float glass has a very smoothsurface, better than 0.1 μM Ra and is widely available in a variety ofthicknesses. In the preferred embodiments the clear float glass sheetwill have a thickness of from 4 mm to 25 mm. Thinner sheets may beemployed but are more liable to break during the grinding process.Thicker shots may be ground if desired. In a second preferred embodimentthe clear flat glass sheet is a rolled wired glass sheet. Safety glazingwith a wire grill insert is widely employed. Rolled wired flat glass isconventionally ground and polished so as to remove the pattern appliedby the roller to produce a clear polished wired glass. In the processesof this invention the clear wired glass is ground under controlledconditions so as to produce a translucent wired glass. The clear wiredflat glass generally has a thickness of from 5.0 to 8.0 mm.

The clear glass sheets will preferably be ground on one surface only.Both sides can be ground if necessary. The desirability of grinding onone or both sides will normally depend upon the intended use of theground glass sheet.

The grinding process reduces the thickness of the glass sheet. Ingeneral grinding one face of a lass sheet will reduce the thickness ofthe sheet by approximately 0.5 mm.

The grinding process of this invention may be carried out by mountingthe clear flat glass sheets upon a suitable flat and level table andpassing them under a grinding head which is fed with a suitable abrasivemedium. The grinding head may conveniently be made from cast iron. Awide variety of materials have been proposed for use as abrasives inlass grinding processes. Any of these abrasives are potentially usefulin the processes of this invention. The most commonly used abrasive andthe preferred abrasive in the processes of this invention is sand. Thesand is fed to the grinding head in the form of an aqueous suspension ofsand.

Normally the grinding operation will be carried out using aprogressively finer abrasive during the process. The coarser moreabrasive particles are fed to the initial stage of the grinding processand are replaced by finer particles as the grinding proceeds. Theprocess may be carried out using two main variants of the productionprocess. In the first variant the clear glass sheet is passed under aseries of grinding heads each of which is fed with a specific grade ofabrasive. The abrasive particles from each stage are recovered, regradedand passed to a subsequent stage until they are too abraded to bc offurther use. In the second variant the glass sheet is placed under asingle grinding head and a progressively finer grade of abrasive is fedto that head as the grinding progresses.

The nature of the abrasive, the pressure on the grinding head and theduration of the grinding process all have a significant effect upon theproperties of the ground glass sheet which is produced. It is within theroutine skill of the art to adjust these parameters in such a way as toproduce the desired product. In particular the roughness of the groundside of the glass may be controlled to fall within the desired region.

The degree of translucency is influenced by the parameters of thegrinding process and the process is controlled so as to produce adesired translucent product. For the intended purpose of privacy glazingthe panels have a milky appearance which prevents an observer seeing anobject on the far side of the glazing unless that object is immediatelyadjacent to the glazing.

The grinding processes of the present invention may impart novelcharacteristics to the surface of the class and lead to the productionof novel glass sheets having new and useful properties.

Ground translucent float glass itself is believed to be novel andconstitutes a further aspect of this invention.

The grinding processes may lead to the production of translucent glasssheet having a transmission of from 60 to 80%. In general glass groundon one side only may have a transmission of 70 to 80% and glass groundon both sides a transmission of 65 to 70%. Moreover the ground glass mayhave a haze value of from 80 to 95% and a clarity of from 5 to 20%.Glass sheets having a haze value of from 80 to 90% and an Ra value ofless than 5.0 μM are believed to be novel and comprise a further aspectof the invention.

The applicants associate these novel characteristics with the nature ofthe surface which is produced by the grinding process. One suchparameter which can be measured and which appears to relate to thesecharacteristics is the density of summits [Sds]; the number of summitsin a unit sampling area. Typically the ground glass sheets of thisinvention have an Sds value of from 500 to 1000 per mm².

A second parameter is the Sa value. Sa is the arithmetic mean of theabsolute values of the surface departures above and below the mean planewithin the sampling areas. The ground glass sheets of this invention mayhave Sa values ranging from 0.3 to 40/μm. The lower values within thisrange, e.g. 0.5 to 2.0/ μm are typical of the novel products of thisinvention, particularly those which have been ground on one surface ofthe glass shot only.

A third parameter which may characterise the ground glass sheets of thisinvention is the Kurtosis value Sku which is a measure of the peakednessor sharpness of the surface height distribution. A perfectly randomsurface which have a Kurtosis value of 3.0. The ground sheets of thisinvention may have a Kurtosis value greater than 3.0, at least in someareas, which value indicates that the ground surface is a relativelyspiky surface.

From another aspect this invention provides a novel method for theproduction of a translucent smooth glass sheet using a glass processingplant which comprises a grinding unit and a polishing unit which ischaracterised in that a sheet of clear flat glass is ground in thegrinding unit and the ground translucent sheet is removed from the plantwithout being subjected to a polishing step. The sheet can be removedfrom the production line prior to the polishing step or it can be passedthrough the polishing equipment whilst that equipment is inactive andcollected at the end of the production line.

When the grinding is complete the ground sheet is removed from thegrinding head, washed and dried. The glass is then available for cuttingand/or shipping. In handling the ground glass care should be taken toavoid marking. The ground surface since such marks are unsightly and canbe difficult to remove. Conventional handling equipment can be employedand does not mark the glass.

The ground glass sheets produced by the processes of this invention mayfind use in a variety of applications. They are potentially useful asreplacements to acid etched glass in most of the applications in whichacid etched glass is currently employed.

The ground glass sheet produced by grinding one surface of a sheet offloat glass finds use as a privacy glass e.g. as a shower screen and inparticular as part of an article of furniture. Typically the groundglass sheets for use in this application may have a thickness of from 3mm to 7 mm. Sheets having a thickness of 5 mm (after grinding) aretypical of those used in this application.

These ground glass sheets may also find application in laminated glazingunits and in double glazing units where a degree of privacy is desired.In this application it may be preferred to position the ground glasssheet so that the ground surface is not exposed to the atmosphere anddoes not attract dirt or other contamination.

Another potential use for the ground glass sheets of this invention isas moulds in the production of acrylic castings. Acrylic castings may bemade by casting a resin onto the surface of a suitable mould and curingthe resin. The cured resin carries any pattern which is on the surfaceof the resin. The glass sheets used in this application are preferablyrelatively thick; say 10 mm or 12 mm thick. Sheets ground on one side ofthe glass only are commonly applied although sheets ground upon bothsides can be advantageously employed in equipment which is designed topermit the production of a cast resin on each side of the glass mouldsimultaneously.

EXAMPLE 1

Plates of float glass 3.3 m×1.985 m×6.3 mm were laid on a series ofmoving grinding tables and separated from the tables by a cotton benchcloth. This glass was ground by passing under a series of seven grindingheads revolving at ˜60 r.p.m, each fed with a sand slurry.

At the first head, the sand particle size was typically 300μ diameterand at each subsequent head the average particle size was progressivelyreduced such that the seventh head was fed with sand particles typicallyof 50μ diameter.

The surface of the float glass was transformed from a fire-polishedsurface better than 0.01 μm Ra to a ‘satin’ finish of surfaceroughnesses of 0.156; 0.062 and 0.072 (measured over three samplelengths) and a roughness of 09±0.1 μm Ra measured by a Rank TaylorHobson Surtronic Four Plus instrument.

Over the same sample lengths the ground surface of the glass had an Sdsvalue of 620, 670 and 610; an Sa value of 3.79, 3.03 and 2.03 μm and anSku value of 2.27, 2.76 and 207. The ground glass had a transmissionfactor of 76%; a haze value of 90.2% and a clarity of 7.5%.

The plates were subsequently removed from the grinding tables forinspection and packaging.

Example 2

Plates of float lass 3.3 m×1.985 mm×6.3 mm were laid on a series ofmoving grinding tables and separated from the tables by a cotton benchcloth, were ground by passing under a series of twelve grinding headsrevolving at 60 r.p.m., each fed with a sand slurry.

At the first head, the sand particle size was typically 300μ diameterand at each subsequent head the average particle size was progressivelyreduced such that the twelfth head was fed with sand particles typicallyof 20μ diameter.

The surface of the float glass was transformed from a fire-polishedsurface better than 0.01 μm Ra to a ‘satin’ finish of surfaceroughnesses 0.107, 0.107 and 0.111 (measured over three sample lengths)and a roughness of 0.4±0.1 μm Ra measured using a Rank Taylor HobsonSurtronic Four Plus instrument.

Over the same sample lengths the rough surface of the glass had an Sdsvalue of 940, 960 and 960, an Sa value of 139. 0.51 and 0.61/μm and anSku value of 1.91, 337 and 3.85. The ground glass had a transmissionfactor of 81.2%; a haze value of 85.9% and a clarity of 12.4%.

The plates were subsequently removed from the grinding tables forinspection and packaging.

Example 3

The procedures of examples 1 and 2 were replicated using rough castwired glass as the starting materials. The products were translucentsheets having one smooth surface. The face which was not ground retainedthe original rough cast surface. In the first case the ground face ofthe glass had a satin smooth finish and a smoothness of 0.4±0.1 μM Raand in the second case the ground face had a satin smooth finish and asmoothness of 0.9±0.1 μM Ra (measured using a Rank Taylor HobsonSurtronic Four Plus Instrument). In the first case the glass had atransmission factor of 69.3%; a haze of 91.0% and a clarity of 8.1 to.In the second case the glass had a transmission factor of 73.5%; a hazeof 84.9% and a clarity of 14.9%.

What is claimed is:
 1. A translucent ground glass sheet having a hazevalue of from 80% to 90% and an Ra value an the ground surfaces of from0.05 μM to 5.0 μM.
 2. The translucent ground glass sheet according toclaim 1 wherein the Ra value on the pound surfaces is from 0.05 μM to3.0 μM.
 3. The translucent ground class sheet according to claim 1 whichis ground upon one side only.
 4. The translucent ground glass sheetaccording to claim 3 wherein the ground surface has density of summitsvalue of from 500 to 1000 per mm².
 5. The translucent ground glass sheetaccording to claim 3 wherein the ground surface has an Sa value of from0.5 to 4.0 μM.
 6. The translucent ground glass sheet according to claim3 wherein the ground surface has a Kurtosis ratio of greater than 3.0.